Flow Patterns in the Carotid Arteries of Patients with Left Ventricular Assist Devices

Development of in vitro microfluidic models to study endothelial responses to pulsatility with different mechanical circulatory support devices

Continuous-flow ventricular assist devices (CFVAD) and counterpulsation devices (CPD) are used to treat heart failure (HF). CFVAD can diminish pulsatility, but pulsatile modes have been implemented to increase vascular pulsatility. The effects of CFVAD in a pulsatile mode and CPD support on the function of endothelial cells (ECs) are yet to be investigated. In this study, two in vitro microfluidic models for culturing ECs are proposed to reproduce blood pressure (BP) and wall shear stress (WSS) on the arterial endothelium while using these medical devices. The layout and parameters of the two microfluidic systems were optimized based on the principle of hemodynamic similarity to efficiently simulate physiological conditions. Moreover, the unique design of the double-pump and double afterload systems could successfully reproduce the working mode of CPDs in an in vitro microfluidic system. The performance of the two systems was verified by numerical simulations and in vitro experiments. BP and WSS under HF, CFVAD in pulsatile modes, and CPD were reproduced accurately in the systems, and these induced signals improved the expression of Ca2+, NO, and reactive oxygen species in ECs, proving that CPD may be effective in normalizing endothelial function and replacing CFVAD to a certain extent to treat non-severe HF. This method offers an important tool for the study of cell mechanobiology and a key experimental basis for exploring the potential value of mechanical circulatory support devices in reducing adverse events and improving outcomes in the treatment of HF in the future.

Study on the hemodynamic effects of different pulsatile working modes of a rotary blood pump using a microfluidic platform that realizes in vitro cell culture effectively

Rotary blood pumps (RBPs) operating at a constant speed generate non-physiologic blood pressure and flow rate, which can cause endothelial dysfunction, leading to adverse clinical events in peripheral blood vessels and other organs. Notably, pulsatile working modes of the RBP can increase vascular pulsatility to improve arterial endothelial function. However, the laws and related mechanisms of differentially regulating arterial endothelial function under different pulsatile working modes are still unclear. This knowledge gap hinders the optimal selection of the RBP working modes. To address these issues, this study developed a multi-element in vitro endothelial cell culture system (ECCS), which could realize in vitro cell culture effectively and accurately reproduce blood pressure, shear stress, and circumferential strain in the arterial endothelial microenvironment. Performance of this proposed ECCS was validated with numerical simulation and flow experiments. Subsequently, this study investigated the effects of four different pulsation frequency modes that change once every 1-4-fold cardiac cycles (80, 40, 80/3, and 20 cycles per min, respectively) of the RBP on the expression of nitric oxide (NO) and reactive oxygen species (ROS) in endothelial cells. Results indicated that the 2-fold and 3-fold cardiac cycles significantly increased the production of NO and prevented the excessive generation of ROS, potentially minimizing the occurrence of endothelial dysfunction and related adverse events during the RBP support, and were consistent with animal study findings. In general, this study may provide a scientific basis for the optimal selection of the RBP working modes and potential treatment options for heart failure.

Stroke in Patients with Left Ventricular Assist Devices

BACKGROUND

Left ventricular assist devices (LVADs) are artificial pumps used in end-stage heart failure to support the circulatory system. These cardiac assist devices work in parallel to the heart, diverting blood from the left ventricle through an outflow graft and into the ascending aorta. LVADs have allowed patients with end-stage heart failure to live longer and with improved quality of life compared to best medical therapy alone. However, they are associated with significant risks related to both thrombosis and bleeding in this medically complex patient population. As LVADs continue to be used more widely, stroke neurologists need to become familiar with the unique physical exam and vascular imaging findings associated with this population.

SUMMARY

Reported rates of LVAD-associated stroke at 2 years post-implantation range from 10 to 30%, which is significantly higher than in age-matched controls. There are approximately equal rates of ischemic and hemorrhagic strokes, and rates are highest during the peri-implantation period and in the first year of therapy. Risk factors associated with ischemic and hemorrhagic stroke in this cohort can be grouped into treatment-related factors, including specific devices and antithrombotic/anticoagulation strategy, and patient-related factors. Evidence for reperfusion therapy for acute stroke in this population is limited. Intravenous tissue plasminogen activator (IV-tPA) is often contraindicated as events may occur in the perioperative setting, or in the context of therapeutic anticoagulation. Endovascular therapy with successful recanalization is reported, but there is little experience documented in the published literature. Key messages: LVAD use is increasingly common. Given the high associated risks of stroke, neurologists will need to become increasingly familiar with an approach to assessment and therapy for LVAD patients with cerebrovascular issues.

Increased pulsatility index is associated with adverse outcomes in left ventricular assist device recipients

Aims

Recipients of left ventricular assist devices (LVAD) are exposed to increased risk of adverse clinical events. One of the potential contributing factors is non‐pulsatile flow generated by LVAD. We evaluated the association of flow patterns in carotid arteries and of increased arterial stiffness with death and cerebrovascular events in LVAD recipients.

Methods and results

We analysed data from 83 patients [mean age 54 ± 15 years; 12 women; HeartMate II (HMII), n = 34; HeartMate 3 (HM3), n = 49]. Pulsatile and resistive indexes, atherosclerotic changes in carotid arteries (measured by duplex ultrasound), and arterial stiffness [measured by Endo‐PAT 2000 as the augmentation index standardized for heart rate (AI@75)] were evaluated 3 and 6 months after LVAD implantation. Sixteen patients died during follow‐up (27.3 months; interquartile range 15.7–44.3). After adjusting for the main variables examined, the pulsatility index measured at 3 months was positively associated with increased hazard ratios (HR) for death and cerebrovascular events [HR 9.8, 95% confidence interval (CI) 1.62–59.42], with HR increasing after adding AI@75 to the model (HR 18.8, 95% CI 2.44–145.50). In HM3 recipients, HR was significantly lower than in HMII recipients (HR 0.31, 95% CI 0.11–0.91), but the significance disappeared after adding AI@75 to the model (HR 0.33, 95% CI 0.09–1.18).

Conclusions

The risk of death and cerebrovascular events in LVAD recipients is associated with increased pulsatility index in carotid arteries and potentiated by increased arterial stiffness. The same risk is attenuated by HM3 LVAD implantation, but this effect is weakened by increased arterial stiffness.

Intraoperative Management of Carotid Endarterectomy in Patients With Left Ventricular Assist Devices-The Challenge of Continuous Flow: A Case Report

Symptomatic carotid artery disease stenosis warrants open surgical carotid endarterectomy (CEA). However, patients with continuous-flow left ventricular assist devices (CF-LVADs) present unique challenges when vasopressors and volume are used to maintain cerebral perfusion pressure after carotid cross-clamping. This report describes patients with CF-LVADs who underwent CEA. We identify how preload, contractility, afterload, pump speed, mean arterial pressure, and anticoagulation should be addressed to maintain CF-LVAD outflow and cerebral perfusion during the procedure. Anesthesiologists can combine an understanding of continuous-flow physiology with invasive monitors to optimize cardiac output and cerebral blood flow during CEA procedures.

Carotid artery duplex velocity criteria might be equivocal after left ventricular assist device implantation

BACKGROUND

Although conventional angiography remains the reference standard for the grading of carotid stenosis, carotid duplex ultrasound (CDUS) is the most commonly used modality for determining the degree of carotid stenosis. The validity of CDUS findings for patients after left ventricular assist device (LVAD) implantation is questionable, because the velocities are often altered secondary to the continuous flow nature of the devices.

METHODS

A retrospective review was performed of all patients who had undergone LVAD implantation from January 2007 to December 2019. All patients who had undergone CDUS before and after LVAD implantation were included. Patients receiving extracorporeal membrane oxygenation, those with unusable carotid imaging studies, and those with internal carotid artery (ICA) occlusion were excluded. The peak systolic velocity (PSV) and end-diastolic velocity (EDV) in the ICA and common carotid artery (CCA) and the ICA/CCA ratios were compared before and after LVAD implantation.

RESULTS

A total of 36 patients (mean age 59 years; 30 men; 6 women) had undergone CDUS both before and after LVAD implantation (mean, 647 days between imaging studies). A total of 61 ICAs had met the criteria for inclusion. Before LVAD, 7 carotid arteries (13%) had had >50% carotid stenosis and 53 (87%) had had 0% to 50% stenosis. The mean changes in the velocities after LVAD were as follows. The ICA PSV had decreased by 6.12 ± 4.34 cm/s, and the ICA EDV had increased by 13.44 ± 4.23 cm/s. The CCA PSV had decreased by 17.22 ± 4.95 cm/s, and the CCA EDV had increased by 10.83 ± 2.59 cm/s. The mean ICA/CCA ratio had increased by 0.18 ± 0.05. All the mean changes in velocity were significant (P < .01), except for the ICA PSV (P = .167). Among four patients with known stenosis of 60% to 69%, the degree of increase in the ICA and CCA EDVs (75.8 and 13.3 cm/s, respectively) was significantly greater than that for patients with <50% or no stenosis. Carotid artery laterality did not significantly affect the differences in mean velocity. Centrifugal LVADs resulted in a significantly larger increase in the ICA EDV compared with axial LVADs (26.0 vs 6.3 cm/s; P < .01).

CONCLUSIONS

LVADs were associated with significant changes in CCA PSV, ICA and CCA EDV, and ICA/CCA ratios. However, the magnitude of these changes in patients with <50% stenosis was minimal and might not be clinically significant. The LVAD type might only have an effect on EDV measurements in the CCA, and the left and right carotid arteries did not appear to have different degrees of change in velocity. The currently used criteria for determining carotid stenosis might result in an under- or overestimation of carotid stenosis in patients with an LVAD.

How Are We Monitoring Brain Injuries in Patients With Left Ventricular Assist Device? A Systematic Review of Literature

Despite the common occurrence of brain injury in patients with left ventricular assist device (LVAD), optimal neuromonitoring methods are unknown. A systematic review of PubMed and six electronic databases from inception was conducted until June 5, 2019. Studies reporting methods of neuromonitoring while on LVAD were extracted. Of 5,190 records screened, 37 studies met the inclusion criteria. The neuromonitoring methods include Transcranial Doppler ultrasound for emboli monitoring (TCD-e) (n = 13) and cerebral autoregulation (n = 3), computed tomography and magnetic resonance imaging (n = 9), serum biomarkers (n = 7), carotid ultrasound (n = 3), and near-infrared spectroscopy (n = 2). Of 421 patients with TCD-e, thromboembolic events (TEs) were reported in 79 patients (20%) and microembolic signals (MES) were detected in 105 patients (27%). Ischemic stroke was more prevalent in patients with MES compared to patients without MES (43% vs.13%, p < 0.001). Carotid ultrasound for assessing carotid stenosis was unreliable after LVAD implantation. Elevated lactate dehydrogenase (LDH) levels were associated with TEs. Significant heterogeneity exists in timing, frequency, and types of neuromonitoring tools. TCD-e and serial LDH levels appeared to have potential for assessing the risk of ischemic stroke. Future prospective research incorporating protocolized TCD-e and LDH may assist in monitoring adverse events in patients with LVAD.

Carotid artery structure and hemodynamics and their association with adverse vascular events in left ventricular assist device patients

Left ventricular assist devices (LVADs) are associated with major vascular complications including stroke and gastrointestinal bleeding (GIB). These adverse vascular events may be the result of widespread vascular dysfunction resulting from pre-LVAD abnormalities or continuous flow during LVAD therapy. We hypothesized that pre-existing large artery atherosclerosis and/or abnormal blood flow as measured in carotid arteries using ultrasonography are associated with a post-implantation composite adverse outcome including stroke, GIB, or death. We retrospectively studied 141 adult HeartMate II patients who had carotid ultrasound duplex exams performed before and/or after LVAD surgery. Structural parameters examined included plaque burden and stenosis. Hemodynamic parameters included peak-systolic, end-diastolic, and mean velocity as well as pulsatility index. We examined the association of these measures with the composite outcome as well as individual subcomponents such as stroke. After adjusting for established risk factors, the composite adverse outcome was associated with pre-operative moderate-to-severe carotid plaque (OR 5.08, 95% CI 1.67-15.52) as well as pre-operative internal carotid artery stenosis (OR 9.02, 95% CI 1.06-76.56). In contrast, altered hemodynamics during LVAD support were not associated with the composite outcome. Our findings suggest that pre-existing atherosclerosis possibly in combination with LVAD hemodynamics may be an important contributor to adverse vascular events during mechanical support. This encourages greater awareness of carotid morphology pre-operatively and further study of the interaction between hemodynamics, pulsatility, and structural arterial disease during LVAD support.

Effects of continuous-flow left ventricular assist devices on cerebral hemodynamics

Continuous-flow left ventricular assist devices (LVADs) reduce peak systolic flow, increase diastolic flow, and eliminate pulsatility of circulation. Altered blood flow may lead to a change in end-organ perfusion. Analysis of the flow dynamics of the arteries of end organs, such as the brain, may indicate whether an organ is perfused sufficiently. The aim of this study is to evaluate and identify the flow pattern changes of carotid (CA) and middle cerebral arteries (MCA) in LVAD patients and to compare with heart failure patients and healthy volunteers. Eighty-nine individuals were included in this cross-sectional study. Participants were divided into three groups: LVAD patients (n = 31), heart failure patients (n = 26), and healthy volunteers (n = 27). Carotid and transcranial Doppler ultrasonography were performed for all study groups for peak systolic velocity (PSV), end-diastolic velocity (EDV), pulsatility (PI), and resistive (RI) indices of CA and MCA. Flow dynamics were compared between the groups. Doppler ultrasonographic data were analyzed at a median 12 (3-47) months after LVAD implantation. CA-PSV was lower in LVAD group compared with the other two groups (P < .001), MCA-PSV of LVAD and heart failure groups were similar and lower than healthy volunteers (P < .05). The highest values for CA-EDV were found in the LVAD group (P < .05). MCA-EDV values were found to be lowest in heart failure group (P < .05). For PI and RI, in all CA and MCA, the LVAD group had lower indices compared with the other two groups (P < .001). In addition, MCA flow analysis in patients with LVADs was identified for the first time with this study.